It is often the case to make the transmission ratio steering wheel rotation/output shaft rotation) to a steering mechanism smaller for sharp handling when turn angle of a steering wheel is small, to make the transmission ratio larger for easy manipulation when the turn angle becomes large, and on the contrary, to make the transmission ratio to the steering mechanism larger for stability in neutral position of the steering wheel when the turn angle of the steering wheel is small. Further, it is the case in garaging to make the transmission ratio small considerably so that the turn angle of the steering wheel can be enough to be small. To cope with these cases, a transmission ratio variable mechanism which is capable of making the transmission ratio to the steering mechanism variable has come to be put into practical use.
By the way, the provision of the transmission ratio variable mechanism on a steering column brings advantages in the respects that there is not required any particular space for the mounting and that the incorporation of the transmission ratio variable mechanism can be realized without changing the construction of the steering mechanism. Regarding a transmission ratio variable mechanism of this kind, there has been known a technology described in Japanese Unexamined, Published Patent Application No. 2003-237590.
In the technology described in Japanese Unexamined, Published Patent Application No. 2003-237590, because with the rotation of the steering wheel, a motor built in a housing of the transmission ratio variable mechanism has to be rotated together with the steering wheel shaft, there is required a spiral cable which allows the motor itself to rotate while supplying electric signals to the motor. For this reason, the transmission ratio variable mechanism in the prior art not only needs an extra space, but also entails a risk of having a drawback such as the breakage of the cable wires.
The present invention has been made in order to solve the foregoing drawback and is intended to provide a transmission ratio variable steering device which is space-saving by eliminating a spiral cable and by arranging a motor on a housing attached to a steering column, at right angles to the axial direction of a steering wheel shaft.